This invention relates to steering control systems for ships, typically oil tankers and other ocean going vessels, which require large forces for steering while at the same time providing precise control for minimum deviation from the set course.
The steering arrangements for large ships require large forces at the rudder post and the ability to control the forces from the remotely located ship's bridge. It has been conventional to utilize a large hydraulic actuator for moving the rudder, and a reversible variable displacement pump to drive the actuator, with a followup control system from the helm at the bridge to the pump. One conventional type of followup system commonly used has a large and expensive differential gearing mechanism and a relatively cumbersome arrangement for transmitting helm commands from the bridge to the rudder drive.
In an alternative approach attempted for the purpose of reducing the cost and complication of the steering arrangement, a three position directional valve has been utilized for controlling the rudder actuator. This type of system is sometimes referred to as a bang-bang system. Only a simple electrical connection is required from the helm to the rudder drive, but there are disadvantages to this configuration including lack of precision in determining rudder position, excessive shock loads, and low durability.
Another type of ship steering control, sometimes referred to as a hydromechanical control, is shown in U.S. Pat. Nos. 3,799,096 and 3,758,235, and in the paper Hydromechanical Differential for Ship Steering System by Robert H. Breeden delivered at the 30th National Conference on Fluid Power, Philadelphia, Pa. Nov. 12-14, 1974. The hydromechanical system does not have the disadvantages of the bang-bang system and eliminates the complicated differential gearing, however it does have other disadvantages.
Steering control systems for ships, while requiring large power output and precise control, should have a slow response time which calls for a low-rate of flow of fluid in the hydraulic controls. Precise control is more difficult to achieve with the low-rate-of-flow systems. This is a particular problem where fluid flow across a variable orifice is utilized as an error signal and non-linear pressure differentials develop. One mode of improving the operation of the hydromechanical system has been to increase the gain of the control system and thereby increase the response at low error signals. However instability sometimes results and is to be avoided.
Accordingly, it is an object of the present invention to provide a new and improved steering control system for a ship which can utilize the conventional power components and which converts the electric signals from this helm directly to a position error within the control component. A further object of the invention is to provide such a steering control system which avoids the differential gearing and bang-bang problems by utilizing hydraulic control and which avoids the problems of the prior art hydraulic systems by incorporating a new and improved control valve arrangement, which generates a differential control pressure by means of a cam. This arrangement allows the gain to be varied by shaping the cam lobe to provide a sensitive and stable system response under all conditions.
Other objects, advantages, features and results will more fully appear in the course of the following description.